This invention relates to a method and a device for detecting crosstrack registration errors in a multicolor printing apparatus and applying correction based on error values.
In the printing industry for multicolor printing, multicolor printing is distinguished in particular by a superposition of individual color separations on an assembly, which are perceived by the human eye on the ultimate stock in various shades, concentrations and brightnesses. It is important that the individual color separations, normally yellow, cyan, magenta and key or black are precisely arranged on top of each other, because otherwise, the ultimate printing may show visible color defects on the stock, i.e., the stacked superposition of the individual color separations affects the printing quality in various ways. Such defects may be manifested in displaced color lines on the printed object. Various methods are thus provided to precisely arrange (in the order of micrometers) the color separations on top of each other. For this purpose, color registration marks are used as reference or comparison indices, which can be used to set chromatic aberrations in a vertical (intrack) or horizontal (crosstrack) direction.
For example, U.S. Pat. No. 5,384,592 contains an image-forming device to transfer a plurality of images, a first image-forming station to form a first registration mark on the image-transferring device and a second image-forming station to form a second registration mark on the image-transferring device. Furthermore, the image-forming device contains an imaging sensor to determine the position of the geometric focal point of the first and second registration mark with respect to an individual spatially fixed reference point, a segment to determine an erroneous alignment of the second image-forming station on the basis of the position of the second registration mark in relationship to the first registration mark, and a segment to correct the erroneous alignment of the second image-forming station. The erroneous alignment is determined in this case by two sensors and an arrangement of each of the two registration marks per color separation.
U.S. Pat. No. 5,287,162 discloses a method and a device for correcting color registration errors. A detection system includes a marking device to apply a chevron of a first printer on a carrier, a second chevron of a second printer and a third chevron from the first as well as the second printer. The third beam has a first component from the first printer and a second element from the second printer. The detection system further contains a detection device for calculating a time matrix with three time slots, whereby each time slot corresponds to a relevant passage of time of the first, second and third chevron through the detection device. The detection system further contains a detection device for determining the alignment error on the basis of a function of three time slots. A control system includes a marking device to apply a chevron of the first printer on a carrier, a second chevron of a second printer and a third chevron from the first as well as from the second printer. The control system further contains a detection device for determining a time matrix with three time slots, whereby each time slot corresponds to a relevant passage of time of the first, second and third chevron through the detection device, a detection device for determining the alignment error on the basis of a function of the time slots and a control device to minimize the alignment error. To this end, the detection device consists of at least two sensors.
One disadvantage of both apparatus described in the above patents is, inter alia, that at least two sensors, as well as a plurality of geometric objects, are used as registration marks for each color separation, in order to measure aberrations of the registration marks.
It is accordingly a purpose of this invention to detect, in a multicolor printing apparatus, crosstrack aberrations of the registration marks in an easy and cost effective way with low use of components and low computer expense independent of intrack aberrations. Registration mark sensor values are advantageously detected by counting cycles with high frequency. A counting cycle to determine a sensor value begins with a light/dark transition of a registration mark and ends with a light/dark transition of the same registration mark lying on the opposite side, i.e., at the edges of a registration mark. Subsequently, a new counting cycle is started, which begins at the edge of the following registration mark at which a light/dark transmission is present and ends on the opposite edge of the registration mark, i.e., the following light/dark transmission. All that is required for a precise detection of a sensor value is the two results of the above-described counting cycle for each registration mark. It thus follows that, in order to detect possible registration errors for each registration mark according to the determination of a differential value from the sensor values, only the two counting results and the differential value are required.
Furthermore, the crosstrack aberrations can be corrected with an advantageous embodiment, whereby first the differential values are determined from the sensor values, by which the differential values of the crosstrack aberrations and the corresponding correction values are calculated and the crosstrack aberrations are ultimately corrected on the basis of the correction values.
The correction values calculated to correct the crosstrack aberrations may be transferred to a control device. For example, the control device is changed by the alignment of a marking device for the application of colored images on a printing drum in such a way that the crosstrack aberrations are essentially corrected in the micrometer range. The result is that the color separations on the individual printing drums with successive printing phases are essentially precisely arranged on top of each other and, as a result, color errors in the final printing are prevented.
The control device for correcting the crosstrack aberrations may be contained in a preferred embodiment in the sensor device. Thus, savings can be made in components and transferal processes. Crosstrack aberrations are calculated in the sensor device and corresponding correction values are transferred to the control device, which controls the alignments of a conveyor belt and/or a marking device of the multicolor printing apparatus.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.